1. Field of the invention
This invention relates to a multi-color developing device for use in an image forming apparatus, and more particularly, it relates to a multi-color developing device for an image forming apparatus in which a plurality of developing units are disposed retractably with respect to a photosensitive drum.
2. Description of the prior art
A conventional two-color developing device for an image forming apparatus comprises: a pair of developing units disposed retractably with respect to a photosensitive drum; a cam mechanism for bringing either one of the developing units to a retracted position away from the photosensitive drum when the other one of the developing units is positioned in a developing position adjacent to the photosensitive drum; a pair of driving units for driving the respective developing units; driving gears provided in the respective driving units; and joint gears provided in the respective developing units and engaged with the corresponding driving gears to transmit power to the respective developing units. The joint gear in each developing unit is constantly engaged with the corresponding driving gear whether the developing unit is in the developing position adjacent to the photosensitive drum or in the retracted position away therefrom. Also, in each of the driving units, a clutch is provided for disengaging transmission of power when the corresponding developing unit is in the retracted position.
In the above-mentioned conventional developing device, each joint gear is put in the proper state of engagement with the corresponding driving gear when the corresponding developing unit is positioned in the developing position adjacent to the photosensitive drum, while these gears engage with each other only by the edges of their teeth when the developing unit is positioned in the retracted position away from the photosensitive drum. That is, the joint gear gradually meshes deeper with the driving gear from the tooth edge engagement into the proper state of engagement as the developing unit leaves the retracted position remote from the photosensitive drum and moves closer to the photosensitive drum into the developing position where the developing roller in the developing unit faces the photosensitive drum with a predetermined clearance.
In the above-described construction, the joint gear and the driving gear must be manufactured so that the predetermined clearance is provided between the photosensitive drum and the developing roller in the developing unit when the gears are properly engaged, thus requiring high accuracy in the finished gears. Also, in the above-mentioned conventional construction, since the gears are constantly engaged with each other, the developing unit can be moved only within the range of the gear engagement, thus limiting the travelling stroke of the developing unit. Furthermore, in the above-mentioned conventional construction, since the joint gear in each developing unit is constantly engaged with the corresponding driving gear, it is necessary to provide a clutch in each driving unit for disengaging transmission of power from the driving unit to the developing unit.